1. Field of Invention
Embodiments of the invention relate generally to operation of uninterruptible power supply systems.
2. Discussion of Related Art
An uninterruptible power supply (UPS) is typically used to provide backup power to an electrical device, or load, while the primary power source, or mains, is unavailable. A conventional online UPS rectifies input power provided by an electric utility using a power factor correction circuit (PFC) to provide a DC voltage to a DC bus. The rectified DC voltage is typically used to charge a battery while mains power is available, as well as to provide power to the DC bus. In the absence of mains power, the battery provides power to the DC bus. From the DC bus, an inverter generates an AC output voltage to the load. Since the DC bus is always powered either by mains or the battery, the output power of the UPS is uninterrupted if the mains fails and the battery is sufficiently charged.
Some conventional UPS systems with three-phase AC input use only a single PFC converter for all three input phases through a front end diode bridge rectifier. As a result, these conventional UPS systems with three phase input have the disadvantage of poor input power factor, high input current distortion and poor efficiency. Further, some conventional high power UPS systems require a split battery input to achieve charging of the negative DC bus, which makes the system costly, less flexible and complex. Additionally, some conventional high power UPS systems do not have flexibility in terms of scaling the output power of the UPS and connecting an appropriate input, e.g., the ability to convert high power with three phase input and low power with single phase input.